chore: Parallelize mutating projects and running the initial test

[richardwerkman]

I'm expecting a modest performance improvement from this on large projects. After this change we:

• Run the initial test while mutating the syntax trees
• Mutate syntax trees in parallel, instead of mutate projects in parallel

This will use up more threads, but reduces the runtime of stryker on multi core systems.

Test results:
I got the initial testing and mutating duration on the integration test project down from 4500ms to 1600ms on a 10 core system.

todo:

• Test on a real world project to measure performance boost
• Delay writing mutated assembly to after inital test is finished
• Fix NUnit and MSTest integration tests

closes #3414

[Copilot]

Pull request overview

This PR aims to improve overall runtime on large solutions by overlapping the “initial test run” phase with the project mutation phase.

Changes:

• Renamed orchestration entrypoint to MutateAndTestProjectsAsync and wired it through StrykerRunner.
• Split initialization into “create inputs” vs “run initial tests”, enabling initial tests and mutation to execute in parallel.
• Adjusted mutator behavior to enrich test-project metadata after initial tests complete, plus updated unit tests for the new flow.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Stryker.Core/Stryker.Core/StrykerRunner.cs	Uses the new orchestrator method that performs parallel initial testing + mutation.
src/Stryker.Core/Stryker.Core/Initialisation/ProjectOrchestrator.cs	Implements the parallel execution of initial tests and mutation, and applies initial-test results after both complete.
src/Stryker.Core/Stryker.Core/Initialisation/ProjectMutator.cs	Moves initial-test enrichment out of MutateProject into a dedicated post-initial-test method.
src/Stryker.Core/Stryker.Core/Initialisation/InitialisationProcess.cs	Splits input creation from running initial tests (now returns a per-input result map).
src/Stryker.Core/Stryker.Core.UnitTest/StrykerRunnerTests.cs	Updates mocks/verifications for the renamed orchestrator API.
src/Stryker.Core/Stryker.Core.UnitTest/Initialisation/ProjectOrchestratorTests.cs	Updates tests to call the new orchestration method and stubs the enrichment call.
src/Stryker.Core/Stryker.Core.UnitTest/Initialisation/ProjectMutatorTests.cs	Ensures enrichment is invoked explicitly after mutation.
src/Stryker.Core/Stryker.Core.UnitTest/Initialisation/InitialisationProcessTests.cs	Updates tests to use the new “inputs then initial tests” API split.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 New Minor Issues (required ≤ 0)

See analysis details on SonarQube Cloud

Catch issues before they fail your Quality Gate with our IDE extension SonarQube for IDE

[Copilot]

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

[Copilot]

The filtering branch calls executedTests.GetIdentifiers() inside the Where predicate, which can cause repeated enumeration and quadratic behavior for ITestIdentifiers implementations backed by non-materialized IEnumerable (e.g., WrappedIdentifierEnumeration). Materialize identifiers once (e.g., to a HashSet) before the Where to avoid repeated enumeration and reduce overhead when many tests are present.

Suggested change

	TestDescriptions = executedTests.IsEveryTest
	? vsTestDescriptions.ToList()
	: vsTestDescriptions.Where(p => executedTests.GetIdentifiers().Contains(p.Id)).ToList();
	var executedTestIds = executedTests.GetIdentifiers().ToHashSet();
	TestDescriptions = executedTests.IsEveryTest
	? vsTestDescriptions.ToList()
	: vsTestDescriptions.Where(p => executedTestIds.Contains(p.Id)).ToList();

[Copilot]

GetMutationTestInputs now runs DiscoverTests in a simple foreach, which makes test discovery strictly sequential across projects. Previously (via the async-per-project input creation) discovery and initial test preparation could run concurrently, which can matter in solution mode with many test projects. Consider parallelizing discovery per project (or integrating it back into RunInitialTestsAsync per-input) so this refactor doesn’t introduce a new bottleneck before the parallel initial-test/mutation phase starts.

Suggested change

	foreach (var project in projects)
	{
	DiscoverTests(project, runner);
	}
	Parallel.ForEach(projects, project =>
	{
	DiscoverTests(project, runner);
	});